Initial dosing regimens of ZDV were based on the plasma half-life of the drug. However, recent studies of the intracellular metabolism of ZDV have demonstrated that the active anabolite, ZDV-TP, is present within the cell for an extended period of time relative to the drug in the plasma. This suggests that antiviral activity may be present for a sufficient time frame with less-frequent dosing of the drug. Careful comparison of the rate and extent of intracellular phosphorylated ZDV metabolites as a function of schedule will determine whether less-frequent dosing has a sound pharmacological basis. Also, the intracellular metabolism of 3TC is via different enzymes than that of ZDV and there are quantitative differences in the amount of triphosphate formed from both drugs. This study will provide information about intracellular metabolites when both ZDV and 3TC are concurrently administered.